my experience with ghost shrimp has been that they have some scavenging value but not nearly as much as scavenging fish. a group of 2 or 3 might make some difference. another observation i would offer is that the ghost shrimp i have purchased have always had a relatively high mortality rate. i don't even bother in larger tanks, something always eats them. in the only tanks i have that i have been able to keep them alive (a well-planted 10g with a frog and a couple guppies and a 29g currently with a dozen cory cats and a pair of honey gouramis), only about half of what i put in survived.
my theory is that only the strongest ones seem to survive tropical temps, a lot of the ghost shrimp commercially available are probably not a true tropical species. i have no evidence on this, there are many subspecies of ghost shrimp from what i understand, but who knows where the shrimp at a random lfs are collected or what conditions they are bred under?
anyway, back to the point...i would say a couple might help a little for the purpose you want, but i would monitor them closely for the first day or two in the tank. if they survive that, they will probably be fine for whatever their life span in aquariums is...i think i read one year or less somewhere?